Replace ClosedWorld.backend with ClosedWorld.backendClasses

pkg/compiler/lib/src/js_backend/lookup_map_analysis.dart

Index: pkg/compiler/lib/src/js_backend/lookup_map_analysis.dart
diff --git a/pkg/compiler/lib/src/js_backend/lookup_map_analysis.dart b/pkg/compiler/lib/src/js_backend/lookup_map_analysis.dart
index 2045c6c6577ba5a93ade038010a88c2d1bbdafe5..d729e561fa7049fb07d27db4c80701d2dd0b8fe0 100644
--- a/pkg/compiler/lib/src/js_backend/lookup_map_analysis.dart
+++ b/pkg/compiler/lib/src/js_backend/lookup_map_analysis.dart
@@ -277,7 +277,7 @@ class LookupMapAnalysis {
         // type_lookup_map/generic_type_test
         // TODO(sigmund): can we get rid of this?
         backend.computeImpactForInstantiatedConstantType(
-            backend.typeImplementation.rawType, impactBuilder);
+            backend.backendClasses.typeImplementation.rawType, impactBuilder);
         _addGenerics(arg);
       }
     }